Method for preparing sandwich type composite membrane capable of efficiently adsorbing heavy metal ions

Abstract

The invention discloses a method for preparing a sandwich type composite membrane capable of efficiently adsorbing heavy metal ions, and belongs to the field of water pollution treatment. The preparation method comprises the following steps: firstly, carrying out hydrolysis modification on polylactic acid particles, then dissolving the polylactic acid particles in dichloromethane / N, N-dimethylformamide to prepare an outer-layer spinning solution, and meanwhile, mixing polyvinyl alcohol, sodium alginate and polyethyleneimine to prepare an inner-layer spinning solution; performing electrostatic spinning on the outer-layer spinning solution to obtain an outer-layer film named as an outer-layer PLA film, performing electrostatic spinning on the inner-layer spinning solution to obtain an inner-layer film named as an inner-layer PVA / SA / PEI film, and re-spinning the outer-layer PLA film on the other side of the inner-layer PVA / SA / PEI film by using the outer-layer spinning solution, namely wrapping the inner-layer PVA / SA / PEI film with the two outer-layer PLA films to form a sandwich structure; and finally, obtaining the composite membrane by adopting a calcium ion cross-linking process. The composite membrane can efficiently adsorb metal ions, is high in adsorption performance retention rate and good in strong acid stability, and is suitable for selective iron removal of electroplating wastewater.

Description

Technical Field

[0001] The invention belongs to the field of water pollution treatment and specifically discloses a method for preparing a sandwich composite membrane for efficiently adsorbing heavy metal ions. Background Art

[0002] With the continuous advancement of industrialization, the pollution problem of heavy metals in wastewater is becoming increasingly serious, not only damaging the quality of water sources, but also seriously affecting the stability of ecosystems. At present, the methods for treating heavy metal wastewater include chemical precipitation, electrochemical method, membrane separation method, ion exchange method, adsorption method, etc. Although these methods can effectively remove heavy metals from wastewater, they still have some disadvantages. For example, chemical precipitation method lacks precision and there is a risk of secondary pollution. Electrochemical method is relatively expensive and electrodeposition has poor separation effect on multi-metal wastewater. Membrane separation method is easily blocked by impurities in wastewater, and its application is limited.

[0003] Adsorption is gaining more and more attention because it combines the physical interception of "membrane separation" with the chemical selectivity of "adsorbent materials". In particular, as the core functional component of wastewater treatment, the adsorption efficiency, selectivity and structural stability of adsorbent materials directly determine the pollution control effect and treatment cost. Although the adsorption method has the advantages of simple operation, low energy consumption and a wide range of applications, problems such as poor selectivity, limited adsorption capacity and poor chemical stability still limit the application of this technology in the field of environmental protection. For example, functional materials can be modified through functionalization, such as the introduction of functional groups such as amino and thiol groups, which can significantly improve the selectivity and adsorption capacity of the adsorbent. However, during long-term use, the functional groups may fall off or become inactivated, resulting in a decrease in adsorption performance. When treating acidic or alkaline wastewater, the membrane material will also undergo hydrolysis or oxidation reactions, reducing its adsorption performance.

[0004] Therefore, if a composite membrane with high adsorption capacity and good chemical stability can be prepared, it will be beneficial to the development of sewage treatment technology. Summary of the Invention

[0005] In order to solve the above-mentioned problems in the prior art, the present invention proposes a method for preparing a sandwich-type composite membrane for highly efficient adsorption of heavy metal ions. The prepared composite membrane can efficiently adsorb metal ions, reach adsorption equilibrium in a relatively short time, and has good environmental tolerance and stability in strongly acidic wastewater.

[0006] In order to achieve the above purpose, the present invention solves the technical problem by adopting the following technical solutions:

[0007] A method for preparing a sandwich composite membrane for high-efficiency adsorption of heavy metal ions comprises the following steps:

[0008] S1. The polylactic acid particles are first hydrolyzed and modified to obtain modified polylactic acid;

[0009] S2. dissolving the modified polylactic acid in a mixed solvent to prepare a 5–10 wt% modified polylactic acid solution, i.e., the outer layer spinning solution;

[0010] S3. The polyvinyl alcohol and sodium alginate were mixed, and polyethyleneimine was added to obtain an inner spinning solution;

[0011] S4. The outer layer spinning solution and the inner layer spinning solution are respectively subjected to an electrospinning process to obtain an outer layer PLA film and an inner layer PVA / SA / PEI film. The inner layer PVA / SA / PEI film is located between the two outer layer PLA films. The film is then subjected to a calcium ion cross-linking process and cured to obtain a sandwich composite membrane.

[0012] The outer layer of the composite membrane obtained by the above method is a PLA fiber layer, and the inner layer is a PVA / SA / PEI fiber layer. The three layers are directly composited into an integrated structure through electrospinning.

[0013] Preferably, the concentration of modified polylactic acid in the outer layer spinning solution in step (2) is 8%, and the volume ratio of dichloromethane to N,N-dimethylformyl is 8:2.

[0014] Furthermore, in the above method, in step S1, the hydrolysis modification is to hydrolyze the polylactic acid particles in a 0.05-0.2 mol / L NaOH solution for 15-60 minutes, wash and dry to obtain modified polylactic acid. Preferably, the concentration of the NaOH solution is 0.1 mol / L and the hydrolysis time is 40 minutes.

[0015] Furthermore, in the above method, in step S2, the mixed solvent is prepared by mixing dichloromethane and N,N-dimethylformamide in a volume ratio of (7-9): (1-3).

[0016] Furthermore, in the above method, in step S3, the volume ratio of the polyvinyl alcohol and sodium alginate solution is (7-9):(1-3); the concentration of the polyvinyl alcohol is 8-12%, and the concentration of the sodium alginate is 1-3%.

[0017] Preferably, the volume ratio of the polyvinyl alcohol to sodium alginate solution is 8:2.

[0018] Furthermore, in the above method, in step S3, the mass ratio of sodium alginate to polyethyleneimine is 1:(1-2).

[0019] Furthermore, in the above method, in step S4, the electrospinning of the outer layer PLA film and the inner layer PVA / SA / PEI film are both carried out at room temperature; the electrospinning conditions of the outer layer PLA film are voltage 14-18 kV, receiving distance 16-20 cm, propulsion speed 2-4 mL / h, and spinning liquid volume 0.5-1.5 mL; the electrospinning conditions of the inner layer PVA / SA / PEI film are voltage 14-18 kV, receiving distance 18-22 cm, propulsion speed 1-1.5 mL / h, and spinning liquid volume 3-5 mL.

[0020] Furthermore, in the above method, in step S4, the crosslinking agent in the calcium ion crosslinking process is a 2% CaCl2 solution, and the crosslinking time is 1-6 hours.

[0021] The present invention also discloses a sandwich composite membrane for high-efficiency adsorption of heavy metal ions, which is prepared by the above method. The fiber diameter of the sandwich composite membrane is less than 200 nm and the porosity is greater than 30%.

[0022] Furthermore, the sandwich composite membrane has a 3+ The adsorption capacity is ≥90 mg / g, and the adsorption equilibrium is reached within 30–50 min under pH 2–6 conditions.

[0023] The present invention also discloses the application of the sandwich composite membrane in the treatment of electroplating wastewater, which is characterized in that it is used for selective adsorption and removal of Fe 3+ ion.

[0024] Compared with the prior art, the present invention has the following outstanding beneficial effects:

[0025] The present invention uses mild and controllable alkaline hydrolysis conditions to modify hydrophobic polylactic acid, significantly improving its hydrophilicity while avoiding the collapse of the fiber structure caused by strong alkaline treatment, effectively maintaining the high porosity and structural integrity of the membrane, and laying the foundation for subsequent efficient adsorption.

[0026] The present invention is made into a sandwich structure through electrospinning process. The outer layer is made by electrospinning the outer layer spinning solution. While maintaining excellent mechanical strength to provide strong support for the whole, its improved hydrophilicity facilitates solution penetration. The inner layer wrapped between the two outer layers is made by electrospinning the inner layer spinning solution. Its rich hydroxyl (-OH), carboxyl (-COOH) and amino (-NH2) functional groups work synergistically to form multi-dentate chelating sites, which significantly enhances the binding of target heavy metal ions (such as Fe 3+ ) adsorption capacity and selectivity. Furthermore, the three layers are directly combined into one through coaxial electrospinning, combined with calcium ion cross-linking and curing, giving the membrane excellent interlayer bonding, effectively overcoming the interlayer delamination problem of the physical stacking structure in acidic environments and significantly inhibiting swelling.

[0027] The composite membrane prepared by the present invention reaches adsorption equilibrium in a relatively short time, exhibits rapid adsorption kinetics, has good environmental tolerance and excellent structural stability in strongly acidic wastewater, and is suitable for selective iron removal from electroplating wastewater. DETAILED DESCRIPTION

[0028] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0029] In the present invention, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Furthermore, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0030] In the present invention, there is no particular limitation on the specific dispersion and stirring treatment methods.

[0031] In the present invention, the test methods used are conventional methods unless otherwise specified; the materials, reagents, etc. used are reagents and materials that can be obtained from commercial channels unless otherwise specified.

[0032] The raw material parameters used in the embodiment of the present invention are shown in Table 1 below:

[0033] Table 1 Raw material parameters

[0034]

[0035]

[0036] Example 1

[0037] A method for preparing a sandwich composite membrane for high-efficiency adsorption of heavy metal ions comprises the following steps:

[0038] (1) Polylactic acid hydrolysis modification: Polylactic acid particles were hydrolyzed in 0.1 mol / L NaOH solution for 40 min, washed with deionized water three times, and vacuum dried at 60 °C for 12 h to obtain modified polylactic acid.

[0039] (2) Spinning solution preparation:

[0040] Outer layer spinning solution: The modified polylactic acid was dissolved in a mixed solvent (prepared by mixing dichloromethane and N,N-dimethylformamide in a volume ratio of 8:2) to prepare an 8% modified polylactic acid solution, and stirred at room temperature and 100 rpm for 4 hours to obtain the outer layer spinning solution.

[0041] Inner layer spinning solution: 10% polyvinyl alcohol and 2% sodium alginate were mixed in a volume ratio of 8:2, polyethyleneimine was added (sodium alginate: polyethyleneimine mass ratio = 1:1.5), and stirred at 80°C and 100 rpm for 6 h to obtain the inner layer spinning solution.

[0042] (3) Electrospinning:

[0043] Outer layer membrane: voltage 16 kV, receiving distance 18 cm, propulsion speed 3 mL / h, spinning solution volume 1 mL, the obtained membrane is named outer layer PLA membrane.

[0044] Inner layer membrane: voltage 16 kV, receiving distance 20 cm, propulsion speed 1.2 mL / h, spinning solution volume 4 mL, the prepared membrane is named inner layer PVA / SA / PEI membrane.

[0045] (4) Cross-linking and curing: The inner PVA / SA / PEI film was placed between two outer PLA films to obtain a sandwich-structured composite film. The composite film was immersed in a 2% CaCl2 solution for cross-linking for 5 h, rinsed with deionized water, and then dried.

[0046] Example 2

[0047] A method for preparing a sandwich composite membrane for high-efficiency adsorption of heavy metal ions comprises the following steps:

[0048] (1) Polylactic acid hydrolysis modification: Polylactic acid particles were hydrolyzed in 0.05 mol / L NaOH solution for 60 min, washed with deionized water three times, and vacuum dried at 60 °C for 12 h to obtain modified polylactic acid.

[0049] (2) Spinning solution preparation:

[0050] Outer layer spinning solution: The modified polylactic acid was dissolved in a mixed solvent (prepared by mixing dichloromethane and N,N-dimethylformamide in a volume ratio of 8:2) to prepare an 8% modified polylactic acid solution, and stirred at room temperature and 100 rpm for 4 hours to obtain the outer layer spinning solution.

[0051] Inner layer spinning solution: 8% polyvinyl alcohol and 3% sodium alginate were mixed in a volume ratio of 7:3, polyethyleneimine was added (sodium alginate: polyethyleneimine mass ratio = 1:1.5), and stirred at 80°C and 100 rpm for 6 h to obtain the inner layer spinning solution.

[0052] (3) Electrospinning:

[0053] Outer layer membrane: voltage 14 kV, receiving distance 20 cm, propulsion speed 4 mL / h, spinning solution volume 1 mL, the obtained membrane is named outer layer PLA membrane.

[0054] Inner layer membrane: voltage 16 kV, receiving distance 20 cm, propulsion speed 1.2 mL / h, spinning solution volume 4 mL, the prepared membrane is named inner layer PVA / SA / PEI membrane.

[0055] (4) Cross-linking and curing: The inner PVA / SA / PEI film was placed between two outer PLA films to obtain a sandwich-structured composite film. The composite film was immersed in a 1% CaCl2 solution for cross-linking for 6 h, rinsed with deionized water, and then dried.

[0056] Example 3

[0057] A method for preparing a sandwich composite membrane for high-efficiency adsorption of heavy metal ions comprises the following steps:

[0058] (1) Polylactic acid hydrolysis modification: Polylactic acid particles were hydrolyzed in 0.2 mol / L NaOH solution for 15 min, washed with deionized water three times, and vacuum dried at 60 °C for 12 h to obtain modified polylactic acid.

[0059] (2) Spinning solution preparation:

[0060] Outer layer spinning solution: The modified polylactic acid was dissolved in a mixed solvent (prepared by mixing dichloromethane and N,N-dimethylformamide in a volume ratio of 9:1) to prepare an 8% modified polylactic acid solution, and stirred at room temperature and 100 rpm for 4 hours to obtain the outer layer spinning solution.

[0061] Inner layer spinning solution: 10% polyvinyl alcohol and 2% sodium alginate were mixed in a volume ratio of 8:2, polyethyleneimine was added (sodium alginate: polyethyleneimine mass ratio = 1:1.5), and stirred at 80°C and 100 rpm for 6 h to obtain the inner layer spinning solution.

[0062] (3) Electrospinning:

[0063] Outer layer membrane: voltage 16 kV, receiving distance 18 cm, propulsion speed 3 mL / h, spinning solution volume 1 mL, the obtained membrane is named outer layer PLA membrane.

[0064] Inner layer membrane: voltage 18 kV, receiving distance 22 cm, propulsion speed 1.2 mL / h, spinning solution volume 5 mL, the prepared membrane is named inner layer PVA / SA / PEI membrane.

[0065] (4) Cross-linking and curing: The inner PVA / SA / PEI film was placed between two outer PLA films to obtain a composite film. The composite film was immersed in a 3% CaCl2 solution for cross-linking for 1 h, rinsed with deionized water, and then dried.

[0066] Comparative Example 1

[0067] The difference between Comparative Example 1 and Example 1 is that, in Comparative Example 1, there is no hydrolysis modification of polylactic acid, and unhydrolyzed polylactic acid particles are directly used to prepare the outer layer spinning solution, and all other conditions are the same.

[0068] Comparative Example 2

[0069] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 adopts a single-layer structure and only performs inner layer spinning. The specific steps are: the inner layer spinning liquid is spun into a single-layer membrane at a voltage of 16kV, a distance of 20cm, a speed of 1.2mL / h, and a liquid volume of 5mL, and the cross-linking and curing are the same as in Example 1.

[0070] Comparative Example 3

[0071] The difference between Comparative Example 3 and Example 1 is that the inner layer of Comparative Example 3 does not contain polyethyleneimine, but only contains 10% polyvinyl alcohol and 2% sodium alginate (volume ratio 8:2), and the other contents are the same.

[0072] Comparative Example 4

[0073] The difference between Comparative Example 4 and Example 1 is that Comparative Example 4 adopts traditional blending modification, and the specific steps are: the hydrolysis-modified polylactic acid and the inner layer spinning solution are blended in a mass ratio of 1:1 to prepare a single spinning solution, and a homogeneous film is spun at a voltage of 16 kV, a distance of 18 cm, and a speed of 2 mL / h. The cross-linking and curing are the same as in Example 1.

[0074] Comparative Example 5

[0075] The difference between Comparative Example 5 and Example 1 is that Comparative Example 5 does not undergo crosslinking and is directly dried after electrospinning, and all other aspects are the same.

[0076] Test Example 1

[0077] The composite membranes prepared in Examples 1-3 and Comparative Examples 1-5 were subjected to Fe 3+ Adsorption kinetics and capacity tests were conducted to verify the efficient adsorption performance of the sandwich structure composite membrane for heavy metal ions.

[0078] Test method:

[0079] Adsorption system construction:

[0080] Simulated electroplating wastewater: FeCl3 solution (pH = 4.0, Fe 3+ Initial concentration 100 mg / L).

[0081] Film sample size: 2×2cm 2 (Examples 1-3 and Comparative Examples 1-5).

[0082] Adsorption process monitoring:

[0083] 1. Immerse the membrane sample in 50 mL of wastewater and shake at a constant temperature of 25°C.

[0084] 2. Take samples at regular intervals (5, 10, 20, 30, 50 minutes) and use UV-vis to detect the remaining Fe 3+ concentration (λ=304nm).

[0085] Calculation of adsorption amount: q t =(C0-C t )V / m(q t : adsorption amount at time t; C0, C t : initial / real-time concentration; V: solution volume; m: membrane dry weight).

[0086] Table 2 Adsorption test results

[0087]

[0088]

[0089] As can be seen from Table 2, Example 1 reaches adsorption equilibrium within 35 minutes, and the saturated adsorption capacity is 94.7 mg / g, which is significantly better than the composite membrane prepared in Comparative Example 1 in which the polylactic acid is not hydrolyzed and modified. At the same time, although Comparative Example 3 is hydrolyzed and modified with polylactic acid, the inner layer does not contain polyethyleneimine. The composite membrane prepared therefrom reaches adsorption equilibrium in 90 minutes, and the adsorption capacity is only 59.8 mg / g. The membrane prepared in Comparative Example 4 is not a sandwich structure, and it reaches adsorption equilibrium in 70 minutes, and the adsorption capacity is lower than that of Example 1, indicating that the unique sandwich structure composite membrane of the present invention can efficiently and quickly adsorb metal ions.

[0090] Test Example 2

[0091] The composite membranes prepared in Example 1 and Comparative Examples 1, 4, and 5 were respectively subjected to acid tolerance and structural stability tests to evaluate the performance retention capability of the composite membranes in a strong acid environment.

[0092] Test method:

[0093] Acidic aging treatment: The membrane sample was immersed in a H2SO4 solution with a pH of 2 and shaken for 24 h (25°C).

[0094] Performance testing:

[0095] Interlayer bonding strength: peel strength tested by universal material testing machine (ASTM D903 standard).

[0096]

[0097] Where S w Indicates the swelling degree, W w Indicates wet weight, W dIndicates dry weight.

[0098] Adsorption retention rate: the same method as in Test Example 1 for measuring Fe 3+ Adsorption amount.

[0099] Table 3: Acid resistance and structural stability test results

[0100] Group Peel strength (N / mm) Swelling degree (%) Adsorption retention rate (%) Example 1 1.82±0.15 28.3 95.6 Comparative Example 1 0.41±0.08* 152.7 38.2 Comparative Example 4 Membrane rupture 206.5 0 Comparative Example 5 0.87±0.12* 63.4 71.8

[0101] As can be seen from Table 3, calcium ion crosslinking enables Example 1 to maintain high peel strength at pH = 2, and compared with Comparative Example 4 of traditional blending modification, the swelling degree is only 28.3%, which is much lower than Comparative Example 4. At the same time, the adsorption performance retention rate is >95%, while the composite membrane of Comparative Example 4 suffers from membrane rupture and severe performance degradation, indicating that polylactic acid hydrolysis modification and calcium ion crosslinking help enhance the acid stability of the composite membrane.

[0102] Test Example 3

[0103] Functional group masking experiments were used to analyze the effects of -OH / -COOH / -NH2 groups in the inner layer of PVA / SA / PEI membrane on the adsorption capacity of the composite membrane.

[0104] Experimental methods:

[0105] OH blocking: boric acid (0.1 mol / L) was added to the inner layer;

[0106] COOH blocking: EDC / NHS esterification blocking;

[0107] NH2 blocking: acetylation with acetic anhydride.

[0108] Adsorption comparison: Fe 3+ Adsorption test (conditions are the same as those in Test Example 1).

[0109] Characterization analysis: XPS analysis of Fe 3+ Binding energy (Thermo K-Alpha).

[0110] Table 4 Adsorption capacity decrease rate

[0111] Treatment Adsorption capacity decrease rate (%) Main binding energy shift (eV) Unprocessed 0 <![CDATA[Fe2p3 / 2:711.5]]> -OH masking 32.7 <![CDATA[Fe2p3 / 2:710.8]]> -COOH masking 41.5 <![CDATA[Fe2p3 / 2:710.2]]> <![CDATA[-NH2 masking]]> 38.2 <![CDATA[Fe2p3 / 2:711.1]]>

[0112] As shown in Table 4, -OH, -COOH and -NH2 groups are used for masking, and all of them have a significant effect on Fe 3+ The adsorption amount decreased, indicating that -OH, -COOH, and -NH2 3+ adsorption has an important contribution, confirming that Fe 3+ Coordination bonds were formed between ions and inner functional groups (especially carboxyl groups), revealing that the ternary composite system improved the adsorption capacity through multidentate chelation.

[0113] Test Example 4

[0114] Actual electroplating wastewater treatment verification and analysis of industrial applications of different composite membranes.

[0115] Verification method:

[0116] 1. Wastewater source: Acid pickling tank wastewater from an electroplating plant (containing Fe 3+ 85.6mg / L, Cu 2+ 12.3mg / L, Ni 2+ 8.7 mg / L, pH=3.2).

[0117] 2. Dynamic filtering test:

[0118] Device: Cross-flow filtration system (membrane area 20cm 2 , pressure 0.1MPa).

[0119] Commercial activated carbon membrane

Norit CA 2020

[0120] Flux maintenance calculation formula:

[0121]

[0122] Where, J: flux; V: filtrate volume; A: membrane area; t: time.

[0123] Ion selectivity coefficient formula:

[0124]

[0125] 3. Detection indicators:

[0126] ICP-MS measurement of heavy metal ion concentration (Agilent 7900)

[0127] Membrane life: adsorption capacity decay rate after 72 hours of continuous operation.

[0128] Table 5 Electroplating wastewater treatment verification results

[0129]

[0130] As shown in Table 5, the Fe 3+ / Cu 2+ The selectivity coefficient is 8.7, which is effective for Fe 3 + The removal rate of Fe is >98%, while the Fe removal rate of the composite membrane prepared in Comparative Example 3 without adding polyethyleneimine is >98%.3+ / Cu 2+ The selectivity coefficient is only 3.2, indicating that polyethyleneimine can significantly improve the selectivity of Fe 3+ At the same time, the selectivity of commercial activated carbon membranes is 3+ / Cu 2+ The selectivity coefficient is 1.8, which is almost non-selective, and the adsorption depends on the specific surface area. The flux is much lower than that of the composite membrane in Example 1.

[0131] In summary, the present invention provides a method for preparing a sandwich composite membrane with high efficiency in adsorbing heavy metal ions. The method comprises the following steps: firstly, polylactic acid is hydrolyzed and modified, and then the hydrolyzed polylactic acid is dissolved in dichloromethane / N,N-dimethylformamide to prepare an outer layer spinning solution. Meanwhile, 10% polyvinyl alcohol, sodium alginate and polyethyleneimine are mixed to prepare an inner layer spinning solution. Then, the inner layer and the outer layers on both sides are directly composited by electrospinning. Finally, a calcium ion cross-linking process is used to obtain a sandwich composite membrane. The obtained composite membrane can efficiently adsorb metal ions, including Fe 3+ The adsorption capacity is ≥90mg / g, and the adsorption equilibrium is reached in a relatively short time. It also shows excellent stability in strongly acidic wastewater and has a high adsorption performance retention rate. When it is applied to the actual electroplating wastewater treatment, it has a good effect on Fe 3+ It exhibits high removal rates and excellent selectivity while maintaining high filtration flux and good long-term operational stability. Furthermore, the membrane's main components (polylactic acid / polyvinyl alcohol / sodium alginate) are all biodegradable, reducing the risk of secondary contamination and offering broad application prospects.

[0132] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for preparing a sandwich composite membrane with high efficiency in adsorbing heavy metal ions, characterized in that: The following steps are involved: S1. The polylactic acid particles are first hydrolyzed and modified to obtain modified polylactic acid; S2. dissolving the modified polylactic acid in a mixed solvent to prepare a 5–10 wt% modified polylactic acid solution, i.e., the outer layer spinning solution; S3. The polyvinyl alcohol and sodium alginate were mixed, and polyethyleneimine was added to obtain an inner spinning solution; S4. The outer layer spinning solution and the inner layer spinning solution are respectively subjected to an electrospinning process to obtain an outer layer PLA film and an inner layer PVA / SA / PEI film. The inner layer PVA / SA / PEI film is located between the two outer layer PLA films. The film is then subjected to a calcium ion cross-linking process and cured to obtain a sandwich composite membrane.

2. The method according to claim 1, characterized in that In step S1, the hydrolysis modification is to hydrolyze the polylactic acid particles in a 0.05-0.2 mol / L NaOH solution for 15-60 min, wash and dry to obtain modified polylactic acid.

3. The method according to claim 1, characterized in that In step S2, the mixed solvent is prepared by mixing dichloromethane and N,N-dimethylformamide in a volume ratio of (7-9): (1-3).

4. The method according to claim 1, wherein In step S3, the volume ratio of the polyvinyl alcohol and sodium alginate solutions is (7-9):(1-3); the concentration of the polyvinyl alcohol is 8-12%, and the concentration of the sodium alginate is 1-3%.

5. The method according to claim 1, wherein In step S3, the mass ratio of sodium alginate to polyethyleneimine is 1:(1-2).

6. The method according to claim 1, characterized in that In step S4, the electrospinning of the outer PLA film and the inner PVA / SA / PEI film are carried out at room temperature; the electrospinning conditions of the outer PLA film are as follows: voltage 14-18 kV, receiving distance 16-20 cm, propulsion speed 2-4 mL / h, and spinning solution volume 0.5-1.5 mL; the electrospinning conditions of the inner PVA / SA / PEI film are as follows: voltage 14-18 kV, receiving distance 18-22 cm, propulsion speed 1-1.5 mL / h, and spinning solution volume 3-5 mL.

7. The method according to claim 1, characterized in that In step S4, the crosslinking agent in the calcium ion crosslinking process is 2% CaCl2 solution, and the crosslinking time is 1-6 hours.

8. A sandwich composite membrane for efficient adsorption of heavy metal ions, characterized in that: Prepared by the method according to any one of claims 1-7, characterized in that the fiber diameter of the sandwich composite membrane is less than 200nm and the porosity is greater than 30%.

9. The sandwich composite membrane according to claim 8, characterized in that The sandwich composite membrane has a great influence on the Fe 3+ The adsorption capacity is ≥90 mg / g, and the adsorption equilibrium is reached within 30–50 min under pH 2–6 conditions.

10. Use of the sandwich composite membrane according to claim 8 in electroplating wastewater treatment, characterized in that: For selective adsorption removal of Fe 3+ ion.

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